I think the differentiator is the quality of “evil” — the joker has a type of evil, with a certain texture and patterns associated with it. Whereas a truly 1D villain will simply take on any kind of evil necessary to fit the moral goal. You don’t need to provide a reason behind it, but you still need an internal consistency, which boring characters generally lack. It should feel like there is a driver, even if you don’t know what it is.